1. Field of the Invention
The present invention relates to a circulating pump for pressurizing and circulating fluid in a system such as a boiler, and more particularly to a circulating pump having a sub-impeller below an impeller of the pump to reduce resistance which occurs at the impeller due to pressure difference between an impeller chamber and an inside of the impeller during operation of the pump so that reduces power loss of a motor and prevents a substance such as mud from entering a stator can.
2. Description of the Prior Art
In general, a circulating pump is equipped in a system such as a boiler for circulating fluid under the action of pressure. Such a circulating pump conventionally comprises a motor acting as a power source for the circulating pump; an impeller which is combined with an output shaft of the motor, rotates together with the motor shaft according to the rotation of the motor shaft, and directly pressurizes fluid; and a pump housing which receives the impeller and includes an inlet passage for allowing fluid to flow into the impeller and an outlet passage for allowing fluid pressurized by the impeller to flow out of the impeller.
In such a prior-art circulating pump, fluid that has flowed into the pump housing through the inlet passage of the pump housing is pressurized by the impeller rotating according to the driving of the motor and flows out of the pump housing through the outlet passage thereof.
FIG. 1 shows a first embodiment for the prior-art circulating pump as described above.
According to FIG. 1, a prior-art circulating pump 1 comprises a motor 10, an impeller 20 and a pump housing 30.
Motor 10 includes a stator 11, a rotor 12 and a motor shaft 13. Stator 11 is fixed inside a motor housing 14. Rotor 12 is fixedly assembled with motor shaft 13 and separated from stator 11 by a stator can 15. One end of motor shaft 13 is supported by a lower bearing holder 16a and a lower bush bearing 17a at the inner bottom of motor housing 14. The other end of motor shaft 13 passes through a motor end shield 18 and is supported by an upper bearing holder 16b and an upper bush bearing 17b at motor end shield 18. Motor shaft 13 is supported by a trust bearing 17c which is to keep the motor shaft 13 from rising in ah axial direction of the motor shaft 13. Between lower bearing holder 16a and stator can 15, an O-ring 19a is provided to prevent fluid from leaking out. Likewise, an O-ring 19b prohibits the leakage of fluid between motor end shield 18 and stator can 15.
Impeller 20 includes an impeller body 21 and a shroud 22. Impeller body 21 and shroud 22 are, in general, connected to each other by an ultrasonic welding. Impeller body 21 is tightly fitted at the other end of motor shaft 13 by a bush 23. At the center of shroud 22 a through hole 24 is formed.
Pump housing 30 is disposed onto motor 10. Pump housing 30 includes an inlet passage 31 and an outlet passage 32 at its right and left sides respectively and an impeller chamber 33 at the center thereof. Impeller 20 is disposed inside impeller chamber 33. A suction ring 34 with a flange 34a and a cylinder 34b is provided at one end of inlet passage 31 of pump housing 30 just above shroud 22 of impeller 20. A fluid guide hole 35 is formed at the center of suction ring 34. Cylinder 34b of suction ring 34 protrudes with a predetermined length into through hole 24 formed at the center of shroud 22 of impeller 20. Between through hole 24 of shroud 22 and an outer wall of cylinder 34b of suction ring 34, and between an upper end of shroud 22 and a lower side of flange 34a of suction ring 34, some measure of clearance is provided. An O-ring 19c is provided to prevent fluid from leaking out between pump housing 30 and motor end shield 18.
According to the prior-art circulating pump 1 constructed as above, when an electric current is applied to stator 11 of motor 10, motor shaft 13 which is rotatably supported against motor housing 14 by means of lower bush bearing 17a and upper bush bearing 17b rotates by an electromagnetic force occurring between stator 11 and rotor 12. Accordingly, impeller 20 which is tightly fitted at one end of motor shaft 13 by means of bush 23 rotates. When impeller 20 rotates, the fluid that has flowed into impeller 20 through inlet passage 31 and suction ring 34 of pump housing 30 is pressurized by impeller 20 and is delivered into impeller chamber 33 and subsequently to outlet passage 32. At this time, some of the pressurized fluid in impeller chamber 33 flows Into stator can 15 through a gap between motor shaft 13 and upper bush bearing 17b. The fluid that has flowed into stator can 15 cools motor 10. The fluid which has entered stator can 15 reversely rises to flow into impeller chamber 33, and thereafter flows out of impeller chamber 33 together with other pressurized fluid therein through outlet passage 32.
However, according to the prior-art circulating pump constructed as above, when circulating pump 1 operates, fluid pressure in impeller chamber 33 is higher than that in inside of impeller 31, thus impeller 20 is forced toward suction ring 34. Since motor shaft 13 fixed at impeller 20 is supported by trust bearing 17c which is to keep the motor shaft 13 from rising in the axial direction, impeller 20 rotates against the force affecting suction ring 34 and therefore some of the rotating power of motor 10 is consumed in overcoming the resistance due to the pressure difference. It is a problem in that such a power loss would result in a pump efficiency to decline.
Also according to prior-art circulating pump 1 constructed as above, if a substance such as mud enters stator can 15 together with fluid through the gap between motor shaft 13 and upper bush bearing 17b, since the gap between motor shaft 13 and upper bush bearing 17b is quite narrow, the substance that has entered thereinto is hardly discharged to impeller chamber 33 again. That is, the substance remains in the stator can 15. At this time, if the substance gets in between motor shaft 13 and lower bush bearing 17a, a lubricative rotation of motor shaft 13 can be obstructed. Especially if pump 1 is not used for a long time, the substance will solidify so the rotation of motor shaft 13 could be impossible.